This invention relates to a transmitted-reference, ultra-wideband (TR-UWB) radio communication system and, in particular, to a scheme for using such a signal as a pilot signal that all radio transmitters in a coverage area can receive and derive time synchronization therefrom.
The use of time division multiple access (TDMA) to synchronize multiple transmitters that wish to make use of the same channel is well-known in the communications art. In the TDMA scheme, multiple transmitters make use of a communication channel serially, each one receiving a time slot during which it has exclusive use of the channel. Another scheme for the coordination of multiple transmitters that is related to TDMA is slotted random access. An example of this type of technique is the slotted ALOHA media access approach. In a slotted random access system, transmitters may use the channel at any time they like, but their transmissions must start at a time determined by a certain clock. This clock determines the slots at which transmissions may take place.
One problem associated with both the TDMA scheme and the slotted random access scheme is the establishment and maintenance of a common time base to regulate the transmission times of the transmitters. In general, some kind of clock has to be distributed to all the transmitters to allow them to transmit at the right time. In some radio-based TDMA systems, this is done by assigning a time slot to the master receiver, during which it broadcasts a message, and having all of the transmitters receive this signal and use its time of arrival at the transmitter as a time mark. Thus, some portion of the channel bandwidth is used to transmit timing information rather than data. This information-bearing bandwidth need not be lost if the timing information can be distributed to the transmitters using some kind of out-of-band channel.
Ultra-wideband (UWB) communications is the name given to a type of radio transmission which works by transmitting pulses; in fact, another name for this type of communications is “impulse radio”. (See M. Z. Win and R. A. Sholtz, “Impulse radio: how it works”, IEEE Comm. Letters, vol. 2, pp. 36-38, February 1988, and L. W. Fullerton, “Spread spectrum radio transmission system”, in U.S. Pat. No. 4,641,317.)
Recently, a new UWB communications scheme, called transmitted-reference, delay-hopped (TR/DH) ultra-wideband, has been invented, as described in copending patent application Ser. Nos. 09/753,443 and 09/974,032. The term “transmitted reference” refers to the transmission and reception of multiple pulses in such a manner that synchronization with the individual pulses is unnecessary. Transmitted reference UWB transmits pulses in pairs, and thereby induces a correlation at the receiver that can be measured by standard means. The term “delay-hopped” refers to a code-division multiple access (CDMA) scheme which uses transmitted-reference UWB.
In addition to the standard ultra-wideband (or “impulse radio”) version of TR/DH, the inventors have invented and experimented with a version of TR/DH that uses wideband noise as a carrier, rather than impulse trains. This version of the invention induces correlation at the receiver by transmitting the sum of two versions of a wideband, continuous noise, separated by a log known to the receiver. This invention has advantages in that the noise carrier may be easier to generate than the impulse train carrier, and it is described in copending patent application Ser. No. 09/974,032. In addition to transmitting information TR/DH UWB can be used to establish a time mark with respect to a clock at the receiver; in this way it can be used to distribute a clock. This application is described in copending patent applications Ser. No. 09/973,140 and Ser. No. 10/125,092.